The present invention relates to improvements in a fuel injection pump of the style shown in DE-AS 1,093,619, DE-AS 1,176,422; DE-AS 1,917,927; and, DE-AS 2,306,007.
Fuel pumps of this type are generally used to carry out the following objectives:
(a) Reduction of the fuel consumption of the internal-combustion engine, for example, of a diesel engine of a vehicle, particularly by a more precise fuel metering compared with a carburetor engine;
(b) Making the combustion in the internal-combustion engine more complete and thus having the engine generate less harmful substances and making it more friendly with respect to the environment;
(c) Making the fuel metering well controllable over wide operating ranges, but being able to require a construction of the pump that is as simple as possible.
These partly opposing requirements cannot all be met completely at the same time. By a skillful selection of parameters during the designing, a compromise must be found that approaches an optimum.
In the injection pump according to DE-OS 32 48 086, a precise control of the start of the injection is achieved between two limit values fixed by constructional limits by a switchable valve. This arrangement ensures an emergency operating capability even if the valve is no longer operable.
It is a disadvantage in the case of this arrangement that a valve-controlled discharge bore is arranged opposite the inflow bore axially in the direction to the upper dead center. Because of the high pressures of up to 1,500 bar, this bore represents a weakening of the wall strength in the most highly stressed part of the piston guide. In addition, the distance of both bores must be fixed within certain limits in order to ensure an optimal operation and not to excessively stress the valve disk. As a result, the shape and the measurements of the whole injection pump are fixed, and the valve arrangement is fairly large.
It is the objective of the invention to provide an improved injection pump of this type having, in addition to less intallation and space requirements, a valve bore that has more space in axial as well as in circumferential direction, therefore avoiding a weakening of the piston guide.
According to the invention, this objective is achieved by providing the control valve bore which connects the bore in which the cylinder travels with the control valve at an axial position below the filling and control bore. The control valve bore is connected with the pump pressure space above the piston by central and radial bore and a ring groove. The closing of the control valve determines the beginning of the axial position of the piston at which injection begins between a earliest possible axial position defined when the piston covers the filling and control bore and a latest possible position when the central and radial bore and the ring groove are disconnected from the control valve bore. The control valve is an electromagnetic valve which is normal biased closed by a spring.
The most important advantages of the invention is that the arrangement of the valve-controlled discharge bore can be provided at the circumference of the piston guide in a spacious way. The discharge bore is therefore embedded in less highly stressed parts of the piston guide wall and not in the high stress pressure space. The start of the fuel injection can be controlled well by the valve that blocks or exposes the discharge bore. The control valve discharge bore is connected with the pump pressure space via the central piston bore, in which case its length may be selected almost arbitrarily. As a result the distance of the valve and its housing from the high-pressure portion of the injection pump, the seals for the valve housing to the pump housing are also further away from the high pressure seals. Thus desired installing requirements can be taken into account within a wide range.
DE-OS 2,430,668 shows a piston for an injection pump that has a surrounding ring groove connected with the front side of the piston via a central bore. However, in this case, only one single inflow and discharge bore that is arranged in the housing wall can be controlled.
The controllable valve is preferably developed as an electromagnetic valve. Thus a simple electric control can be achieved. In addition, the valve is preferably a normally closed valve spring, changeable from its position that blocks the fuel discharge. Thus, extremely short closing times can be achieved. Only these short valve closing times permit a continuous change of the start of the injection while the pump piston moves between the upper edge of the inflow or filling and control bore and the upper edge of the discharge or control valve bore. The control edges of the piston are generally selected such that the piston reaches the control valve bore or covers its partially when, in the case of the latest possible start of the injection, the valve body is shifted in the working position. The supplying of the fuel takes place irrespectively of whether the control valve is opened or closed, as soon as the bore is closed completely.